The present invention relates to a mechanism for adjusting the gap between a platen and print head in a printer, and in particular, to an apparatus for adjusting the gap between the platen and print head in response to an output from a paper thickness detector.
Generally, the print density of wire dot impact printers or thermal transfer printers changes in accordance with the gap between a print head and print paper. Therefore, the newest prior art printers detect the thickness of the print paper and move the carriage on which the print head is mounted to adjust the gap between the print head and print paper and optimize print density.
A conventional printer which adjusts the gap between the print head and the print paper is shown in Japanese Patent Laid-Open Application No. 2344872/85 in which a printer includes a pressure sensitive element secured to a carriage. The carriage is movable in a direction orthogonal to the length of the platen. An electric signal indicating the thickness of the print paper is generated by the pressure sensitive element in response to movement of the carriage and the gap between the print head and the platen is automatically adjusted in response to the electric signal.
This device has been satisfactory, however when the thickness of the print paper is detected, the platen must be moved in the vertical direction relative to the print head so that the response speed is low. Additionally, because the relative position between the platen and print head is detected through movement of carriage, measuring errors due to movement of the carriage may be generated when detecting thickness of the print paper. Accordingly, the prior art device suffers from an inferior reliability in adjusting the gap.
Accordingly, it is desirable to provide a mechanism for automatically adjusting the gap between the platen and print head in a printer which overcomes the shortcomings of the prior art.